1. Field of the Invention
The present invention relates to a color electrophotographic method and apparatus which can be applied to apparatus for producing hard copies of color images, such as color copiers, color printers and so forth. More specifically, the present invention is directed to a color electrophotographic method and apparatus such as a light source scanning-exposure type in which a series of operations including steps for charging, exposure and development is conducted cyclically so as to form a plurality of toner images of different colors on an electrophotographic photosensitive medium (hereinafter referred to as a photosensitive medium) and these toner images are transferred in one step onto a sheet of paper.
2. Description of the Prior Art
Such a known color electrophotographic method (types of which are disclosed in, for example, the specifications of Japanese patent Laid-Open No. 95456/1985 and U.S. Pat. No. 4,599,285) has certain disadvantages in that (1) the boundaries of images of different colors which are formed adjacent to each other may be blurred or the width of previously formed color image forming lines may be decreased when the next color image is formed, and (2) the toner particles forming the images may be scattered and the entirety of the images blurred when the electrostatic latent images on the photosensitive medium carrying the toner images are erased by light irradiatied onto the photosensitive medium.
The disadvantages of the conventional apparatus to be overcome will be hereinunder described in detail by referring to FIGS. 4a to 4f which illustrate the process of forming second toner images on a photosensitive medium carrying first toner images that have been formed in the previous process.
The photosensitive medium 2 which carries the first toner images 1 formed by a toner of an opaque color such as black is charged a second time to a surface potential of Vs by a corona charger 3 (FIG. 4a). Next, the areas D and F which are adjacent to the toner image on the area E are exposed (as shown by the arrows) (FIG. 4b) to form electrostatic latent images, so that the surface potential of these areas is attenuated to that of the residual potential (Vr) of the photosensitive medium (FIG. 4c). This generates a large difference in the potentials at the boundaries between the area E and the areas D and F, and the toner particles located in the vicinity of the boundaries are thereby scattered along the lines of electric force which are directed toward the areas D and F from the edges of the area E, thus making the width of the toner image 1 smaller.
As a consequence, when the electrostatic latent images are developed by a second toner 4 of a color which is different from that of the first toner 1 (FIG. 4d), the color purity of the second toner images is degraded by the first toner particles 1 that have scattered to the vicinity of the boundaries of the area E.
Further, when light is irradiated over the entire surface of the photosensitive medium 2 (FIG. 4e) after the development by the second toner 4 so as to attenuate the surface potential of the photosensitive medium 2 at the areas on which no toner is attached (areas A and C and the edges of the area E) to Vr (FIG. 4f), the first toner particles located in the vicinity of the edges of the areas B and E are scattered to the areas A and C and the areas D and F, respectively, for the same reason as that described with reference to FIG. 4b, thereby blurring the images. At this time, the surface potential of the photosensitive medium 2 under the first toner images 1 becomes Vb because it is slightly attenuated by the light passing through the gaps formed between toner particles, even though the toner itself does not transmit light.
The present inventors have carried out intensive studies on the above-described problems, and have found that the scattering of toner particles occurs when the difference between the potentials of the toner image area and the adjacent areas reaches a certain value or is above such value. It has also been found that the toner scattering which occurs when the electrostatic latent images are erased by light occurs to a greater extent as the opacity of the toner increases.